1. Field of the Invention
The present invention relates to an apparatus and a method for Direct Current (DC) offset calibration in a signal transmission device of a wireless communication system.
2. Description of the Related Art
Generally, in a wireless communication environment, a Complementary Metal-Oxide Semiconductor (CMOS) Radio Frequency Integrated Circuit (RFIC) serves to transmit a baseband modem signal using a radio wave, and serves to deliver a reception signal received through an antenna to a Modulator/Demodulator (MODEM) without distortion.
A 4th generation wireless communication standard, such as Long Term Evolution (LTE) or Mobile WiMax, uses a modulation scheme, such as a 16 Quadrature Amplitude Modulation (16QAM) scheme or a 64QAM scheme, in order to transmit and receive a relatively large amount of data without distortion. The use of this modulation scheme indicates that an Error Vector Magnitude (EVM) performance corresponding to a quality index of a signal transmitted by a signal transmission device must be good.
Major factors which affect EVM performance include the linearity of an output-side circuit, the phase noise of a Local Oscillator (LO) signal, the amount of imbalance of an In-phase/Quadrature (IQ) channel, the amount of LO leakage (for example, a DC offset), etc. Particularly, LO leakage is generated by a DC offset of an analog baseband signal which is input to a mixer, and must be minimized in order to improve the EVM performance. Moreover, if the amount of LO leakage increases, even when a signal reception device demodulates a signal transmitted by the signal transmission device, the EVM performance is degraded.
In order to efficiently operate a wireless communication network, the signal transmission device has a dynamic range of 70 to 80 dB, and 30 to 40 dB within the dynamic range of 70 to 80 dB is determined as a dynamic range of an analog baseband. Accordingly, in order to meet the specifications of LO leakage in the entire range of an output voltage (output power), a DC offset must be removed for each gain step in the analog baseband.
FIG. 1 is a block diagram schematically showing an internal configuration of a signal transmission device in a wireless communication system according to the related art.
Referring to FIG. 1, the signal transmission device may include a Digital-to-Analog Converter (DAC) 111, a Current-to-Voltage Converter (IVC) 113, a Variable Gain Amplifier (VGA) 115, a Low Pass Filter (LPF) 117, a DC calibration unit 119, a mixer 121, a Local Oscillator (LO) 123, and a drive amplifier 125.
The DAC 111, which is typically part of a MODEM, first converts an I (in-phase) signal and a Q (quadrature) signal into an analog signal, and outputs the converted analog signal to the IVC 113. The IVC 113 converts the converted analog baseband signal, which is output from the MODEM, into a voltage signal, and the converted voltage signal to the VGA 115.
The VGA 115 amplifies the signal, which is output from the IVC 113, by a previously-set gain, and a signal from the VGA 115 is output to the LPF 117. At this time, the DC calibration unit 119 first removes a DC offset from the signal, which is output from the VGA 115, by using a DC calibration value determined according to the control of the controller, and outputs a signal, which is obtained by removing the DC offset, to the LPF 117. The LPF 117 filters a signal obtained after the DC calibration unit 119 removes the DC offset in such a manner as to limit a bandwidth of the signal to a previously-set bandwidth, and outputs the filtered signal to the mixer 121.
The mixer 121 mixes the signal, which is obtained after the DC calibration unit 119 removes the DC offset, with a signal, which is output from the LO 123, and outputs a mixed signal to the drive amplifier 125. The drive amplifier 125 amplifies the signal, which is output from the mixer 121, by a previously-set gain, and outputs an amplified signal.
As described above, a DC calibration value used by the typical DC calibration unit 119 is a value used to remove a DC offset in the entire analog baseband with respect to a particular gain. Accordingly, when gains used by the VGA 115 and the LPF 117 change, a DC offset is again generated.
Therefore, a need exists for an apparatus and method for DC offset calibration in a signal transmission device of a wireless communication system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.